Gypsum is Calcium Sulphate [CaSO4.2H2O] and is an industrial by-product arising from the manufacture of Phosphoric acid.

Why would we use Gypsum?

When marshes are flooded with sea-water, salt is left behind when the flood-water drains. The sodium in the salt adsorbs onto the clay particles in the soil and also inside the clay-lattice. This sodium causes water to be attracted to the clay particles by osmosis and thus causes the soil particles to swell and disperse. The structure of the soil then collapses ['disperses'] causing poor drainage.

Application of Gypsum is the fastest way to make sure that the clay particles do not disperse. The calcium in the gypsum somehow holds the clay particles together. Gypsum is soluble so the effects can wear off after a number of years [eg 5].

Very fine lime works more slowly but can hold the soil structure together for upto 30 years when applied at upto 30t/ac! The very-fine lime needs to have come from the beet factory or from a water works. Ground limestone is ineffective at preventing soil dispersion. [Bryan Davies Former ADAS Chief Soil Scientist 4/3/99]

How Does Gypsum Work?

When Gypsum is added to dispersed [poorly-flocculated] soils, the calcium in the gypsum, displaces the sodium which had adsorbed on the soil particles. The displaced sodium chemically combines with the sulphate [from the gypsum] to form a sodium sulphate solution. This is lost in the drainage water.

When the calcium from gypsum replaces sodium on the soil particles, water is no longer attracted to the soil particles and they revert to their original size. The soil flocculates [that is, forms little crumbs] and drainage is improved.

Recommending for Gypsum

• Gypsum is applied at between 2-5 tonnes per acre [5-12 t/ha] depending on the severity of dispersion.
• The Big A spreader can apply this and we supply on a delivered and spread basis.
• We can only sell gypsum in full 25-tonne loads.

Recommendations can be fine-tuned by soil analysis: We need to take a standard soil sample and ask for two additional tests from the laboratory

• Exchangable Sodium Chloride
• Conductivity

Soil remediation with gypsum

The following taken from the July 2001 Edition of the FACTS Fertiliser Matters information Sheet

Several questions on use of gypsum have prompted the re-issue of this article by Keith Goulding of IACR Rothamsted. The article first appeared in the 3rd issue of ‘A Matter of FACTS’ in 2000.

Gypsum (calcium sulphate) has traditionally been used to improve the structure of soils. The calcium supplied in gypsum increases the ability of soil particles to flocculate (group together to form aggregates) and so maintain structure. The opposite tendency, deflocculation of particles and loss of structure, occurs for example where soils have been flooded by seawater or brackish water containing high concentrations of sodium. The sodium replaces calcium ions on the soil cation exchange sites. The sodium ion, with its single positive charge, is much less effective than the calcium ion, with a double positive charge, at holding soil particles together.

The failure to maintain soil structure at the microscopic level will affect the larger scale properties of the soil so that the establishment of a good seedbed will be difficult, sometimes impossible, without remediation. Lime and gypsum have been used as remediation agents since the 1950s. Visible improvement to soil structure in the Essex marshes and Wash silts was achieved by application of gypsum. where flocculation of the soil particles has occurred. The application accelerated the removal of excess sodium from the soils after seawater flooding.

The rate of gypsum application should be decided on the basis of soil analysis for salt concentration. Usually, 5 to 10 t/ha are sufficient but there are reports of applications of 50 t/ha resulting in improved soil structure on silty clay loams near Kings Lynn. These reports also suggest that there is a benefit from lime or gypsum applications on neutral or alkaline soils even where the soils contain only a little exchangeable sodium. This effect is generally considered unlikely and there can be problems associated with excessive application of lime or gypsum. Firstly, these can result in loss of availability of most micro-nutrients. Secondly, application of 50 t/ha of gypsum is equivalent to around 23 tSO3/ha (about 450 x the amount needed to correct a crop sulphur deficiency). On grassland, such applications could raise the sulphur content of the herbage to a level where copper deficiency in livestock is induced or exacerbated.

Product Safety

Product Safety Data Sheet Group 8